A traditional mechanical bearing uses a liquid lubricant in combination with ball bearings to reduce the frictional wear of a rotating shaft. However, when used in high precision, high RPM applications, traditional mechanical bearings have several shortcomings. Ball bearings are not perfectly spherical and the race within which they sit can deform slightly. This can cause random fluctuations in the orientation and position of the rotating shaft. Furthermore, mechanical bearings often wear in ways that increase the magnitude of these random displacements. In addition, wear particles and lubricant can spread from the mechanical bearing and contaminate the device incorporating the rotating shaft.
Because of the above difficulties, a gas bearing is often the preferred way of supporting a rapidly rotating shaft for some applications. Gas bearings may use either hydrostatic or hydrodynamic principles of operation. In a hydrostatic bearing a source of pressurized gas is supplied between a rotating shaft and its surrounding sleeve. The gas acts as a lubricant and allows the shaft to rotate without coming into contact with the sleeve. In a hydrodynamic bearing, oblique grooves are cut in a shaft and the rotation of the shaft causes gas to flow through the grooves. The dynamic pressure created by this gas flow allows the gas to act as a lubricant. In many applications, the lubricant gas used is simply filtered air. For other applications, noble gases or nitrogen may be preferred because of their relative inertness.
The pressure created by hydrodynamic gas bearings can be sizable. However, this pressure is usually unavailable for other uses within a device. Any attempt to bleed gas flow from a gas bearing would seriously affect its ability to function properly. Therefore, devices that incorporate hydrodynamic gas bearings are unable to utilize the pressurized gas created by the rotating shaft. If such a device requires a long-term source of pressurized gas it must incorporate a mechanical compressor. These compressors are often undesirable since they occupy space within the device, consume power, and are a source of vibration and contamination.
For the foregoing reasons, there is a need for an apparatus that allows the gas pressure created by a hydrodynamic bearing to be utilized for other purposes by the device incorporating the bearing.